The highest priority is probably making real debugging tools. Right now, the only decent debugging tool is ghc-debug to connect to a live process, and doing anything over that connection is slow, slow, slow. ghc-debug was the only thing which was able to resolve a long standing thunk leak in one of my systems, and I know that unexplained thunk leak caused a previous startup I was at to throw away their Haskell code and rewrite it in Rust. In my case, it found the single place where I had said `Just $` instead of `Just $!` which I had missed the three times I had inspected the ~15k line program. ghc-debug still feels like a pre-alpha though, go compare it to VisualVM for what other languages have.
Also, I have found very little use for the various runtime flags like `+RTS -p`. These flags aren't serious debugging tools; I couldn't find any way to even trigger them internally at runtime around a small section, which becomes a problem when it takes 10 minutes to load data from disk when the profiler is on.
The debugging situation with Haskell is really, really bad and it's enough that I try to steer people away from starting new projects with the language.
Also, I have found very little use for the various runtime flags like `+RTS -p`. These flags aren't serious debugging tools; I couldn't find any way to even trigger them internally at runtime around a small section, which becomes a problem when it takes 10 minutes to load data from disk when the profiler is on.
The debugging situation with Haskell is really, really bad and it's enough that I try to steer people away from starting new projects with the language.